From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=91.218.175.223 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793857741; b=jsMWBy6LAqrx0GswK9Oo8PRe1D9cyomuE1rQatoH1fPrbj4fN/Ykg9YXjsMkNPgnqkbB RUt39ebpcjALR1dfDDXNZgpCjh1ZIcjYuzQDTPbQkwFxajvcdpTEP8P5iszfGuG0oYKrX SEmhBifogZ6q8Ushn9cn2jMuJQkMfuY9KWHN/Poy8h/fwvThB0A/Q9UBecWq7XGHJW2SK 14Epcti7xiuitZ1RtVjdaWDBj+RBmuhmFWy8sF3EkrXRUT0l+UEqLZyQOq6r4Us9itkGZ kKhjraSlVlvFN0dTXMLs9cmn+pRk2TeUwug02XBvtu5iAO5ld61T9mJMymmO18lyXcg== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793857741; h=DMARC-Filter:X-Envelope-To:DKIM-Signature:X-Envelope-To:Received: X-Mizu-Trace-ID:X-Migadu-Flow:Message-ID:Date:MIME-Version:User-Agent: Subject:To:Cc:References:Content-Language:From:In-Reply-To: Content-Type:Content-Transfer-Encoding; bh=/l6O9V7fMoYs7ihQIlg7jRbOXL/S2bFnQjGLeMo1S78=; b=fTmG9P+S5PoNjEm5BCf2M4xDtg9Tp/alnjMq9j9aQQzf6RFh5O4ubI1TiFI7WaWYMNGj zO83ZJ0drDpJYepn71W8W1JoqvHrhjLQer5Vu/wr9kjKSy28klZ1KjjhfGCxg/5u+McSI UyD1DJblWrR3zCCTaNC/2GRwyIvu/WIA1vhfX+MvetmRtCD1cIXQOE7XLJwggg88cbcOG EQWPn0KETASezFU/65voYTr6bZ+iq8biDpL3rvR0NQL6bkELdeXZywSLdIPJAUuE3s3T9 7gWMQg9TM5QOe32vYjG0CQsIqbN5W8wJNXyfj5FX+HX07MjbcOeT7sYCbqpIrHlVLxw== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass header.d=linux.dev header.i=@linux.dev header.a=rsa-sha256 header.s=key1 header.b=JpMvdsIx; arc=none smtp.remote-ip=91.218.175.223 Received: from mta0.migadu.com (out-223.mta0.migadu.com [91.218.175.223]) (using TLSv1.3 with cipher TLS_AES_128_GCM_SHA256 (128/128 bits) key-exchange X25519 server-signature ECDSA (prime256v1) server-digest SHA256) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 3C412606A339 for ; Thu, 5 Nov 2026 05:48:56 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.2 smtp.subspace.kernel.org 3C412606A339 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key, unprotected) header.d=linux.dev header.i=@linux.dev header.a=rsa-sha256 header.s=key1 header.b=JpMvdsIx X-Envelope-To: linux-kernel@vger.kernel.org DKIM-Signature: a=rsa-sha256; bh=l/klO32YzidFN2v1G4JzAIJs9vATWMhL2+jw9rYo5BY=; c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1793857735; v=1; x=1794462535; b=JpMvdsIxUYKjtKIiudsiyW9Sq9GKK1KU4Zx9S9a6W3ZxxzfjNmy/uAkswDpc/2CnYHA6KnI7 rDnABgQXVr1PgaVyvm5LudW0X6ui3shigSzStXvnUFAyMUzIO4y9veyCYQzX25M3yJGZcvOfMS6 5nTGhjYDggi2xM9o/B7q1fCg= X-Envelope-To: linux-kernel@vger.kernel.org Received: by smtp.migadu.com with ESMTPS id 3f45584444935ba1; Thu, 05 Nov 2026 05:48:54 +0000 X-Mizu-Trace-ID: 3f45584444935ba1 X-Migadu-Flow: FLOW_OUT Message-ID: <0ddaf336-7cb7-4e83-af2c-f60d23f7aafd@linux.dev> Date: Thu, 5 Nov 2026 13:48:42 +0800 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm: synchronize with PTE table reclaim for all folio sizes To: baolin.wang@linux.alibaba.com Cc: akpm@linux-foundation.org, david@kernel.org, ljs@kernel.org, riel@surriel.com, liam@infradead.org, vbabka@kernel.org, harry@kernel.org, jannh@google.com, qi.zheng@linux.dev, pfalcato@suse.de, ayushr@modal.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20261105045406.39871-1-lance.yang@linux.dev> Content-Language: en-US From: Lance Yang In-Reply-To: <20261105045406.39871-1-lance.yang@linux.dev> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 2026/11/5 12:54, Lance Yang wrote: > > On Thu, Nov 05, 2026 at 10:56:28AM +0800, Baolin Wang wrote: >> Similar to commit 243cf1f67b36 ("mm: memory: fix truncation removing mapped >> folios"), the '!pmd_present()' check in page_vma_mapped_walk() can also race >> with concurrent page table reclamation via zap_empty_pte_table(). > > Good catch! > >> page_vma_mapped_walk() is used by many functions including try_to_unmap(), >> try_to_migrate(), folio_mkclean(), pfn_mkclean_range(), remove_migration_ptes(), >> page_mapped_in_vma(), and mapping_wrprotect_range(). Except for try_to_unmap() >> and try_to_migrate(), the other callers do not perform post-walk accounting >> checks, so the race has no observable impact on them. >> >> For migration, the race can cause unnecessary retries. For reclaim via >> try_to_unmap(), the race can cause failure to reclaim a folio that is in >> fact already unmapped (as shown below), which may lead to unexpected OOM >> or increased refaults. >> >> CPU 0 (reclaim) CPU 1 (MADV_DONTNEED) >> -------------------------------- -------------------------------- >> folio_lock(F) >> try_to_unmap(F, TTU_SYNC) >> zap_pte_range(full PMD range) >> spin_lock(ptl) >> clear PTEs >> zap_empty_pte_table() >> spin_trylock(pml) >> pmd_clear() >> spin_unlock(pml) >> >> page_vma_mapped_walk() >> observe !pmd_present(pmde) >> nr_pages < HPAGE_PMD_NR >> skip synchronization >> return false >> >> try_to_unmap() returns >> folio_mapped(F) == true >> nr_unmap_fail += folio_nr_pages(F) >> tlb_flush_mmu_tlbonly() >> tlb_flush_rmaps() >> remove F's rmaps >> spin_unlock(ptl) >> >> Commit 243cf1f67b36 ("mm: memory: fix truncation removing mapped folios") >> has already moved the PMD clear operation (via zap_empty_pte_table()) to >> after tlb_flush_rmaps(). Similarly, page_vma_mapped_walk() also needs to >> use sync_with_folio_pmd_zap() to synchronize and avoid memory ordering >> issues on weakly ordered CPUs. >> >> Fixes: 6375e95f381e ("mm: pgtable: reclaim empty PTE page in madvise(MADV_DONTNEED)") >> Cc: stable@vger.kernel.org >> Suggested-by: David Hildenbrand (Arm) >> Signed-off-by: Baolin Wang >> --- >> mm/page_vma_mapped.c | 5 +---- >> 1 file changed, 1 insertion(+), 4 deletions(-) >> >> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c >> index e6b6170c2f4f..8797467b6f0c 100644 >> --- a/mm/page_vma_mapped.c >> +++ b/mm/page_vma_mapped.c >> @@ -293,10 +293,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) >> spin_unlock(pvmw->ptl); >> pvmw->ptl = NULL; >> } else if (!pmd_present(pmde)) { >> - if ((pvmw->flags & PVMW_SYNC) && >> - thp_vma_suitable_order(vma, pvmw->address, >> - PMD_ORDER) && >> - (pvmw->nr_pages >= HPAGE_PMD_NR)) >> + if (pvmw->flags & PVMW_SYNC) > > Right. Whether the folio can be PMD-mapped doesn't tell us whether its > PTE table can be reclaimed, so those extra checks can skip synchronization > that the caller explicitly asked for ;) Ouch ... one thing I missed ... what about order-0 reclaim? shrink_folio_list() only sets TTU_SYNC for large folio, so can't we still hit the same race there? Cheers, Lance > LGTM, thanks! > > Reviewed-by: Lance Yang > >> sync_with_folio_pmd_zap(mm, pvmw->pmd); >> >> goto next_pmd; >> -- >> 2.47.3 >> >>